Pyometra is the most prevalent reproductive disorder threatens dogs and cats' life. This study aimed to address the relation between pyometra, and oxidative stress and hormonal milieu. Bitches (n=200) and queens (n=281) admitted to veterinary clinics from which ten animals per each were pyometric. Health examination, ultrasound investigation and blood sampling were accomplished, besides histopathology of uterus upon ovariohysterectomy. Sera were assayed for inflammatory marker or predictive marker (C-reactive protein; CRP), oxidative stress or risk factors (glutathione peroxidase (GPX), catalase (CAT), total antioxidant capacity (TAC), nitric oxide (NO), superoxide dismutase (SOD), lipid peroxidation (MDA)), and hormones (progesterone (P4), estrogen (E2)). Animals with pyometra showed fever, polydipsia, polyuria, vomiting and purulent vulvar discharge. Ultrasound examination revealed the presence of anechoic to hypoechoic uterine fluid with an increased wall thickness. There was a significant increase in CRP (P< 0.01 and 0.05), NO (P< 0.05), MDA (P< 0.01 and 0.005), P4 (P< 0.05 and 0.0001), P4/E2 ratio (P< 0.05) and a decrease in SOD (P< 0.0005 and 0.005), CAT (P< 0.05), and TAC (P< 0.05), in bitches and queens, respectively. Histopathology showed a suppurative inflammation and heavy leucocytic infiltration in all uterine layers, and severe degenerative changes in endometrial glands with neutrophils-infiltrated homogeneous eosinophilic substances in the lumen. The ultrasound morphometric measurements (uterine diameter, wall thickness, and lumen and wall echo-pattern) were positively correlated with CRP and NO, and negatively correlated with TAC. In conclusion, pyometra greatly influenced the health status and reproductive efficiency of bitches and queens through altering oxidative defense mechanism.